The invention relates to data communication systems, and more particularly to a wireless local area network (LAN) communication system in which a number of remote units associate with stationary access points in a manner that reduces traffic on the LAN.
A wireless LAN is implemented by integrating wireless communication with wired communication. Typically, wireless mobile units (MUs) send messages to and receive messages from Stationary Access Points (SAPs). Each SAP covers a particular geographical area. The network may be extended by connecting each SAP to a host that coordinates message traffic between SAPs.
The IEEEE 802.11 standard is part of a family of standards that define physical and data link layers for wireless local and metropolitan networks. IEEE 802.11 is the standard that defines the wireless LAN medium access control (MAC) and physical layer (PHY) specifications. More particularly, the IEEE 802.11 standard provides MAC and PHY specifications for fixed, portable, and moving mobile units within a local area.
The standard covers both portable and mobile remote units in a wireless LAN environment. A portable remote unit is only used from a fixed location, whereas a mobile remote unit access the LAN while in motion.
Under the standard, the mobile and portable MUs communicate with SAPs using wireless communication. The SAPs may communicate with each other directly, or through one or more host systems. The combination of remote units, stationary access points, and hosts form a network.
As mobile and portable MUs (mobile units) move, SAPs receive and transfer messages from and to the MUs. Each SAP covers a geographical area in which units may communicate with it. In the 802.11 standard, connections between units and stationary access points are handled by a set of Association services.
For a message to be transmitted, an MU must become associated with a SAP. A distribution service on the local area network uses these MU-SAP mappings to transmit messages between the MUs. At any point in time, an MU can be associated with only one SAP. A single SAP can be associated with multiple MUs.
"Association" refers to the process of synchronizing an MU with an SAP for communication, and is initiated by the MU. The MU first determines which SAPs are present, and then requests association with a particular SAP. To determine which SAPs are present, the MU carries out a process called "scanning."
MU scanning can be passive or active. Passive scanning in the IEEE 802.11 standard is performed by MUs monitoring beacon frames sent out by SAPs. Beacon frames are used in the standard to synchronize to the SAPs, and MUs look for these frames to determine which SAPs are close enough to communicate with.
In active scanning, an MU transmits probe frames. All SAPs which can hear this MU receive the probe frames and return probe response frames if certain criteria regarding the probe frame are met.
When a connection with an SAP is impeded, such as by SAP failure, MUs in the geographical region serviced by the failed SAP sends out probe frames to find a new SAP to handle communications. Therefore, there is a spike in the number of probe frames when connection to multiple MUs is impeded, such as when an SAP fails. There is also a spike in the number of probe response frames. This causes the system to be flooded with probe frames and probe response frames, and the system becomes congested. Congestion results in lost communication because the system cannot handle the traffic.
U.S. Pat. No. 5,657,317 to Mahany et al. describes a system for detecting loss of connection, and reconnecting to a different stationary access point. None of the described approaches discussed, however, provide a solution to the problem of eliminating probe frame congestion.
What is needed, then, is a system and method for eliminating probe frames when connection between an SAP and MU is impeded.